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General Characteristics of Phylum Echinodermata

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General Characteristics of Phylum Echinodermata General Characteristics of Phylum Echinodermata (Echinos: Spines; derma: Skin) Phylum Echinodermata forms the highly advanced group among invertebrates.The examples of this group are star-fishes, sea-urchins,sea-cucucmbers etc.This phylum is charecterised by the following salient features, 1. Habitat and Habit: Echinoderma are exclusively marine. They generally live at sea bottom borne are pelagic (free swimming in open water) and a few are sessile (attached to the substratum). 2. They are Multicellular organisms with organ Grade of organisation 3. Blasticity: They are Triploblastic animals 4. Coelom: They are coelomate animals 5. Symmetry: The symmetry is bilateral in larvae but pentamerous radial in adults i. e., body parts are arranged in fives or multiples of five. 6. They have well developed endoskeleton formed of spines and pincer-like pedicellariae. The spines are protective in function. The pedicellariae keep the body surface clear of debris and minute organisms. 7. Ambulacral System (= Water Vascular System): Presence of ambulacral system is the characteristic feature of phylum echinodermata. A perforated plate called madreporite is present in this system. The pores of the madreporite allow water into the system Tube teet of this system help in locomotion, capture of food and respiration. Water vascular system is of coelomic origin. 8. Circulatory system is open type and includes haemal and perihaemal systems. The so called blood is often without a respiratory pigment. There is no heart. 9. Respiratory Organs: Gaseous exchange occurs by dermal branchae or papulae in star fishes peristominal gills in sea urchins, genital bursae in brittle stars, and cloac respiratory ‘trees in holothnrians. Exchange of gases also takes place through tnbe feet. 10. Excretory Organs: Specialized excretory organs are absent. Nitrogenous wastes are diffused out via gills. Ammonia is chief excretory matter. 11. Nervous System: It consists of a nerve ring and radial nerve cords. Brain as such is absent. 12. Sense Organs: They are poorly developed. 13. Sexes and Fertilization: Except a few individuals, the sexes are separate. There is no sexual dimorphism. Fertilization is usually external. 15. Asexual Reproduction: Some forms reproduce asexually by self-division. 16. Autotomy and Regeneration: Phenomena of autotomy and regeneration are often well marked in echinoderms. 17. Development: The development is indirect and includes a ciliated, bilaterally symmetrical larva that undergoes metamorphosis to change into the radially symmetrical adult. Water vascular system videos https://www.youtube.com/watch?v=fcILzBZ_Aj4 https://www.youtube.com/watch?v=K2G7L5hcEt8 CLASSIFICATION OF PHYLUM ECHINODERMATA Phylum Echinodermata is divided in to 2 subphylum 1. Subphylum Eleutherozoa: Three classes-Asteroidea, Ophiuroidea, Echinoidea 2. Subphylum Pelmatozoa: One class with living organisms, Crinoidea Sub phylum 1. Eleutherozoa 2. Pelmatozoa Class Asteroidea Ophiuroidea Echinoidea Holothuroidea Crinoidea Sea star brittle stars and (Gk. echinos- hedgehog, Sea cucumber Sea lilly or sea feather (Gk. aster- star, eidos- basket stars (Gk. eidos- form) (Gk. Holothurion- (Gk. Crinon- lily, eidos- 1. Common form) Ophis - snake, sea cucumber, form) name Oura- tail, eidos- form): eidos- form) Body is flattened star Body is flat with Body is disc-like or oval Body shape varies, Body is star shaped shaped with five arms pentamerous disc or heart shaped may be elongated 2. Body which are not sharply like cucumber or shape marked off from the rounded or five central disc sided Presence of They possess a They possess compact They have no arms, Arms are branched. Spines calcareous plates and long arm which is skeleton and movable spines and and pedicellariae and 3. movable spines. sharply demarcated spines. No free arms pedicellariae. madreporite are absent Spines/arms from the central disc Oral and aboral Oral and aboral Mouth on the oral and Mouth and anus at Mouth is present in the surfaces are quite surfaces are quite anus on the aboral the opposite ends centre and food grooves distinct. Mouth on the distinct. Anus is surface. Biting and radiate along the oral 4. Mouth oral and anus on the wanting chewing apparatus with surface of arms aboral surface teeth called Aristotle’s Lantern is present Class Asteroidea Ophiuroidea Echinoidea Holothuroidea Crinoidea Ambulacral grooves are Ambulacral grooves are Ambulacral grooves are The water vascular Crinoids have no on the oral surface. absent. The madreporite absent. The madreporite system has no madreporite. Instead, Water enters the system is located on the located within one of the connection to the the oral surface is through a sieve-like underside of the animal, plates surrounding the anus outside, and is thus dotted within 5.Water structure on the upper usually in one of the jaw on the upper surface of the filled with the numerous vascular system surface of the animal, plates animal. The stone canal internal coelomic fluid, minute ciliated funnels called the madreporite. descends from the rather than sea water. that run into the main madreporite to the ring The madreporite is body cavity canal present They possess tube feet They possess tube feet They possess tube feet The tube feet are They have tube feet with a sucker without sucker with a sucker. sucking type which is without suckers 6.Tube feet modified into tentacles and form a circle around mouth Larval forms are Larval form is Larval forms are Platens Larval forms are Larval form is 7. Larva Bipinnaria and Ophiopluteus and Echinopluteus Auricularia Doliolaria Brachiolaria Asterias (Star fish), Ophiothrix (brittle star), Echinus (Sea Cucumaria (Sea Antedon (feather star). Astropecten, Ophioderma (brittle star) urchin), Cidaris, cucumber), Holothuria, Sea lilies 8. Examples SEA STAR Phylum: Echinodermata Class: Asteroidea Genus: Asterias Asterias is commonly known by the name of starfish. The name starfish is somewhat misleading suggesting an organism to be like a star and fish. Therefore, recently it is renamed as sea star. Habit and Habitat of Asterias: Asterias is exclusively marine, bottom dwelling or benthonic animal, inhabiting in the bottom of sea. Most of them are nocturnal, remain quiet in day time and become active during night. They move by crawling on the bottom, mostly at a rather slow rate. All sea stars are carnivorous and feed voraciously on almost any available slow moving or sessile animals, chiefly on polychaetes, crustaceans, molluscs and other echinoderms. External Features of Asterias: (i) Shape, Size and Colour: Asterias has a radially symmetrical and pentamerous body. The body consists of a central, pentagonal central disc from which radiate out five elongated, tapering, symmetrical spaced projections, the rays or arms.The size varies from 10-20 cm in diameter though some forms may be much smaller or longer. The colour is variable having shades of yellow, orange, brown and purple. The body has two surfaces, the upper convex and much darker side is called the aboral or abactinal surface. The lower surface is flat, less pigmented and is called the oral or actinal surface. (ii) Oral Surface: The lower of body, which remains towards the substratum and contains the mouth or oral opening, is flat and of dark orange to purplish colour, is oral actinal surface. The oral surface bears the following structures: 1. Mouth: On the oral surface, in the centre of the pentagonal central disc is an aperture, the actinosome or mouth. Mouth is with five angles, each directed towards an arm. The mouth is surrounded by a soft and delicate membrane called peristome and is guarded by five groups of oral spines or mouth papillae. 2. Ambulacral Grooves: From each angle of the mouth radiates a narrow groove called the ambulacral groove which runs all along the middle of oral surface of each arm. 3. Tube Feet or Podia: Each ambulacral groove contains four rows of locomotory, food capturing, respiratory and sensory organs called tube feet or podia. The tube feet are soft, thin- walled, tubular, retractile structures provided with terminal discs or suckers. The suckers function as suction cups to afford a firm attachment on the surface to which they are applied. 4. Ambulacral Spines: Each ambulacral groove is bordered and guarded laterally by 2 or 3 rows of movable calcareous ambulacral spines which are capable of closing over the groove. Near the mouth, these spines often become larger, stouter, assemble in five groups, one at each inter-radius of disc and are called mouth papilla. Outside the ambulacral spines are three rows of stout immovable spines, beyond which occurs another series of marginal spines along the borders of the arms demarcating the oral from the aboral surface. 5. Sense Organs: Sense organs include five unpaired terminal tentacles and five unpaired eye spots. The tip of each arm bears a small median, non-retractile and hollow projection, the terminal tentacle. It acts as a tactile and olfactory organ. At the base of each tentacle occurs a bright red photo-sensitive eye spot made up of several ocelli. (iii) Aboral Surface: The upper convex surface is called aboral or abactinal surface. The aboral surface bears following structures: 1. Anus: A minute circular aperture, called the anus, is situated close to the centre of the central disc of aboral surface. 2. Madreporite: At the aboral surface of the central disc occurs a flat, sub-circular, asymmetrical and grooved plate called madreporite plate or madreporite between the bases of two of the five arms. The surface of madreporite is marked by a number of radiating, narrow, straight or slightly wavy grooves with pores in them. The madreporite is a sieve-like porous plate and it leads to the stone canal of water vascular system. The two arms having madreporite between their bases are collectively referred to as a bivium and the other three arms as a trivium. The symmetrical position of madreporite, thus, converts the radial symmetry of Asterias into bilateral symmetry. 3.
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